Estimating pool capacity

In addition to the space it consumes in a VOB database, each element requires space in the VOB’s source pool and usually other pools.

About this task

Every version of every element in the VOB is stored in a container file in a source pool. Different element types have different type managers. Most type managers compress source data so that each version consumes less storage in the source pool than it does in cleartext form. Type manager compression ratios lists compression ratios for commonly used built-in type managers. (For more detailed information on type managers and element storage capacity, refer to technote 129851.)
Table 1. Type manager compression ratios
Type manager Type description Compression ratio
whole_copy Whole copy of object 1:1
text_file_delta Interleaved deltas of printable text 50:1
z_whole_copy Compressed whole-copy 2.5:1
z_text_file_delta Compressed text_file_delta 200:1

With the information provided in Type manager compression ratios, you could compute a rough approximation of VOB source pool size by using this formula:

P = N * V * S / C

where:
  • P is the source pool size in bytes.
  • N is the number of elements in the VOB.
  • V is the average number of versions per element
  • S is the average file size of each cleartext element version.
  • XSC is the compression ratio of the element’s type manager.

Example

The following example uses this formula to estimate the size of the source pool of a VOB that contains 500 elements of type text_file_delta, with an average of 20 versions per element and an average version size of 50000 bytes:

500 * 20 * 50000 / 50 = 10,000,000 bytes

For VOBs that contain elements of different types, you must repeat this calculation for each element type.

Elements also require space in the cleartext and DO pools, which are caches that are periodically scrubbed to control their growth. Because space requirements of cleartext and DO pools are determined by site-specific patterns of use, there is no general rule for estimating their size based on the number of elements in a VOB.

For more information about VOB storage pools, see VOB storage pools.